1. Field of the Invention
The present invention relates to an illumination device for illuminating display elements and the like.
2. Description of the Related Art
Reflective type liquid crystal display is an example of a display element used in display devices. An advantage of reflective type liquid crystal display as a display element is the large aperture efficiency compared to transmission type liquid crystal display. Attention has been particularly focused in recent years on reflective type liquid crystal display comprised of cholesteric liquid crystal display, which have many advantages including a high reflective efficiency, and the capability of being integratedly formed with a driver IC on a silicon substrate. Display devices which use a reflective type liquid crystal display as a display element are used in calculators, projectors, and head-mounted displays and the like, and card calculators and the like do not require special illumination when used in bright areas. However, illumination methods do pose problems for the various display devices which require other illumination.
A projector is the subject of the example described below. It is desirable to illuminate the display element from directly in front to achieve illumination with the greatest efficiency. When an illumination device is disposed directly in front, however, the light reflected from the display element is blocked by the illumination device and cannot be observed. To prevent this obstruction, conventional devices achieve illumination from the front by using a half mirror or polarization beam splitter.
An example of a display device which achieves illumination from the front surface of a display element via an illumination device using a half mirror is shown in FIG. 16. The light emitted from an illumination lamp 101 is reflected by a reflector 102, and impinges a half mirror 103. The light reflected by the half mirror 103 illuminates a reflective type liquid crystal display element 104. Then, the light is converted to a projection image based on the projection image signals via the reflective type liquid crystal display element 104, and is transmitted through the half mirror 103 and projected to the pupil of an observer via an enlarging lens 105. In this device, the illumination device comprises the illumination lamp 101, the reflector 102, and the half mirror 103. This illumination device achieves illumination from the front surface of the reflective type liquid crystal display element 104 without blocking the projection image light.
Display devices which achieve illumination from the front surface of a display element using a half mirror or polarization beam splitter as described in the conventional art have expensive and complex structures, and are neither compact, nor light weight. Since compactness and light weight are important factors, this construction is unsuitable for display devices, particularly head-mounted displays.